The performance of a hearing aid depends, among other things, upon the design of the microphone assembly which includes the microphone transducer, sound port, and a housing containing the signal processing electronics. The microphone transducer is typically a variable capacitor or electret type microphone formed of a charged diaphragm forming one plate of the capacitor and a backplate forming the other terminal. Sound impinging on the diaphragm varies the capacitance and produces a voltage signal proportional to the sound waves which is picked off the backplate and coupled to signal processing circuits where it is amplified in an amplifier and electrically processed to, inter alia, reduce noise content. The processed signal is then coupled to a receiver and converted back to sound waves to aid the user.
Conventional in the ear (ITE) or in the canal (ITC), hearing aids must of necessity be of relatively small size. Therefore, such aids have been fabricated with accessible replaceable batteries which are accessed via a faceplate door on the hearing aid enclosure. These size and battery requirements cause the microphone assembly and also the diaphragm to be relatively small in size in relation to the size of the hearing aid faceplate. The small diaphragm size lowers the quality of the transducer function.
An electret microphone for hearing aids typically uses a Junction Field Effect Transistor (JFET) buffer to convert the voltage signal from the high impedance transducer source to a low impedance source. This impedance conversion typically requires a difficult connection to be made to a high quality and hence, expensive substrate on a Printed Circuit Board (PCB) containing the signal processing components, so as to avoid compromising the input impedance of an amplifier on the substrate.
This invention is directed to a microphone assembly for a hearing aid comprising a metal housing with a front wall with sound openings and a side wall extending longitudinally away from the front wall. Within the housing is an electret type microphone or transducer having a diaphragm electrode and a backplate electrode. External sound entering through the openings are converted into an electrical voltage signal which is coupled from the backplate to a Junction Field Effect Transistor (JFET) buffer device. The buffered signal is then coupled to an amplifier and signal processing components within the housing.
In one embodiment of the invention, the JFET device is a flip-chip component with four active terminals. Drain, source, bias and gate terminals are provided. The gate terminal is located on a side of the flip-chip proximal to and adjacent the backplate. The other terminals are connected to respective traces on a PCB. All the signal processing circuits needed to provide a functional hearing aid are contained on the PCB. The PCB also provides an acoustic seal to a back volume of the microphone and contains an electromagnetic interference (EMI) ground shield in the form of a ground plane of conductive material extending across the side wall of the housing.